The present invention relates to a new and improved construction of monitoring apparatus for a textile machine, comprising a feeler coupled with an evaluation circuit, the feeler in turn embodying two electrodes arranged in spaced relationship from one another, each of which delivers an electrode signal when the textile material moves therepast.
Such monitoring apparatuses serve to detect irregular operating conditions at textile machines and for automatically triggering a warning signal and/or control operations which eliminate such irregular operation. Under the term "irregular operating condition" or equivalent expressions there is to be understood, for instance, the rupture of, or other irregularity existing at, a yarn, a silver, a slubbing or a fleece, whereas examples of a textile machine with which the invention can be employed are a card, a drafting arrangement, a spinning machine or a loom.
It should be understood that not every signal transmitted by the electrodes is indicative of an irregular operating condition of the machine falling within or without a tolerable range. In order to differentiate which ones of the signals delivered by the electrodes in fact indicate an actual fault or defect, different monitoring apparatuses of the previously mentioned type have already become known to the at which essentially differ from one another by the criteria according to which the evaluation circuit takes into account or does not take into account the signals delivered thereto.
For instance, with the evaluation circuit described in German Pat. No. 1,685,886 the signals delivered by the feeler are analysed with respect to their peak and frequency (repetition) in such a manner that there are only evaluated signals whose amplitude exceeds a minimum value and which are repetitive at a minimum frequency over a longer period of time or are present over a longer period of time. With this state-of-the-art monitoring apparatus which has been found to be satisfactory in practice, the parameters (e.g., minimum peak of the electrode signals, lower boundary frequency, minimum duration of the individual signals, flank shape of the electrode signals) which are to be accommodated to the specific field of application are so numerous that it is not always possible to detect by means of the monitoring apparatus only the significant irregular operating conditions. In other words: with the heretofore known monitoring apparatus an ideal setting or adjustment is hardly possible, so that as a general rule the monitoring apparatus is much too sensitive. Other examples of monitoring apparatuses functioning in this manner have been disclosed in Swiss Pat. No. 540,848.
According to another comparable monitoring apparatus, as disclosed for instance in German Pat. No. 1,139,997, the signals delivered by the feeler are only evaluated with respect to their amplitude in the manner that when and as long as there is present a signal amplitude exceeding a predetermined value there occurs the formation of a corresponding long square-wave pulse which is then differentiated and the resultant differentiated square-wave pulse is employed for triggering a monostable multivibrator possessing sufficiently long return time, which then in turn is employed for controlling an indicator or means for eliminating the irregular operating condition. Also this monitoring apparatus undertakes a selection of the signals which are taken into account only on the basis of the attained amplitudes. Hence, this equipment is not capable of eliminating electrode signals of sufficient amplitude which result for reasons other than the movement of textile material past the electrodes, such as for instance transient, induced electrical voltage peaks in the power supply network or in the field surrounding the electrodes.
In order to overcome such drawback there has been proposed a further construction of monitoring apparatus, which is not part of the state-of-the-art, wherein each of both electrodes, in principle, is coupled with the one input of a differential amplifier, and only the output signal of such differential amplifier is delivered for further processing. This monitoring apparatus is based upon the assumption that textile material moving past the electrodes in any event will not produce the same signal -- as concerns duration and amplitude -- at both electrodes, whereas the signals induced by outside influences, so-called external or spurious signals, in any case are congruent both with respect to amplitude and also duration. Yet, this assumption is not always valid, particularly if it is realized that, for instance, the capacitance of the individual electrodes with respect to ground cannot be assumed to be constant as a function of time.